In controlling corrosion of pipelines or elongated metal structures buried underground or under water cathodic protection techniques have been employed. The earth or water is an electrolyte. To determine where cathodic protection should be applied and to assure that sufficient cathodic protection voltage is applied to the pipeline, a pipeline survey may be made by taking electrical measurements of the pipe-to-soil potential difference and/or soil resistivity at selected locations along the length of the pipeline. The data gathered in such survey can be analyzed to determine where and/or how cathodic protection can be efficiently employed to prolong the life of the pipeline.
The pipe-to-soil or water potential difference measurement requires a contact to the pipe, a suitable voltmeter or potentiometer, a means of contacting the electrolyte, and connecting wires. A copper-copper sulfate (Cu--CuSO.sub.4) cell is an industry standard for providing the necessary contact with the electrolyte. Contact to the pipe usually is provided by a wire connection to a test lead, which is permanently connected to the buried pipe and is brought above ground in a protected, easily accessible location. Such test leads usually are installed along the pipeline from about one to two miles apart. Measurements are commonly made on a yearly basis at the test lead stations to obtain general information concerning the pipeline condition and its relation to the surrounding environment.
However, to obtain more complete data of the pipeline condition a more comprehensive continuous, over-the-pipeline, closely-spaced survey occasionally may be conducted to measure the potential difference, for example, at intervals of, say, 10 to 50 feet along the length of the pipe.
In the past, various techniques have been used to make such relatively closely spaced surveys. In one technique a reel of relatively heavy insulated wire was connected to the pipeline at a test lead, and the wire was dragged from the reel across the ground along the route of the pipeline. The copper-copper sulfate half cell was placed directly over the pipeline at intervals of, say, 10 to 50 feet, and both distance and potential difference measurements were taken and manually recorded. In this technique measurement inaccuracies have occurred due to static electricity accumulation on the dragged wire. Alternatively, the wire was attached to the test lead and the reel was transported by vehicle along the pipeline route. In both cases, though, some form of vehicular transportation was required for the reel and usually powered equipment was required to rewind the wire back onto the reel. Such a system, however, has a number of disadvantages. The reel transporting vehicle and the power-rewinding equipment are heavy, expensive, and consume energy, such as fuel and/or electric power. Several workers were usually required. Since many surveys are made over farm land, crops, etc., and in rough terrain, such as in rocky, mountainous, or wooded areas, across flowing streams, fences, and like impediments, the use of a vehicle often is prohibited or impossible. Also, the physical effort required to drag the heavy wire is considerable, especially when a mile of wire is manually pulled across uneven terrain. The resultant wear on the wire and frequent breakages, plus electric reel maintenance, further add to the cost of such prior systems.
In a recently improved technique for making such relatively closely-spaced surveys a dual function economically disposable, relatively lightweight, flexible wire provides both electrical connection to the pipeline via a test lead and accurate distance measurement information to the surveyor moving along the length of the pipeline. A reel of such lightweight wire is carried by the surveyor, who may walk along the length of the pipeline, and the wire drives a distance measuring unit carried by the surveyor to display the distance from the test station. The surveyor also carries a copper-copper sulfate half cell, which is placed in contact with the ground at selected test locations, and a meter for measuring the potential difference between the wire and the half cell. A single surveyor thus makes both the distance and potential measurements and may write the values in a notebook, verbally recor them on a portable tape recorder, or verbally transmit them by radio for recording at a different location. The wire used for connection to the test lead is not dragged over the ground; rather it is merely laid down as the reel is easily transported along the path of the pipeline. Moreover, the wire is economically disposable and need not be rewound for re-use.
In another recent technique, the supply of wire, distance measuring equipment and electrical measuring equipment are carried by a boat above a submerged pipeline.
The portability of the equipment used in such improved technique facilitates the making of closely-spaced surveys, e.g. by reducing manpower and/or equipment costs over the first-mentioned technique. Also, since the equipment used is highly portable, it can be carried by a single surveyor, for example, even over rough terrains and those over which vehicular travel is prohibited. However, although having a number of advantages over the former techniques described above, nevertheless the distance and electrical information usually is visually read and/or manually recorded.
In addition to the physical problems inherent in older techniques, perhaps the most important consideration with regard to this type of survey is the accuracy of the data collected. The pipeline chaining was often in error from several factors. These would include stretching of the measurement wire from starting and stopping for each reading and inaccurate methods for following the topography of some areas. Also, the value of the galvanic potential developed between the reference electrode and the pipe is one that is much more difficult to measure accurately than is obvious from a superficial understanding. This voltage value can be subject to a high resistance contact at the electrode/earth interface. Therefore, unless a suitable high input impedance voltage measuring instrument is used, many readings would have significant errors. Traditionally, or up to the last two or three years, Potentiometric voltmeters or high resistance D'Arsonval meters have been used. Whatever, they both are quite inferior when compared to contemporary electronic voltmeters (10 meg-ohms are higher input impedance). Another factor that contributed to the voltmeter measurement error is static or friction EMF's generated as the contact wire is pulled along the ground. The polarity of these charges can be positive or negative, but in either case distort the real galvanic voltage.